Immigrants are always being told by politicians to learn the language. But how long does it take to speak good English?

There are plenty of people in the UK for whom even basic English is a problem. According to the Census, 726,000 people in England and Wales said they could not speak English well, and another 138,000 said they did not speak it at all.

Ling, 40, who arrived five years ago from China, found it difficult to learn English. "When I came here I was pregnant and so I was at home for the next three years. It took me longer to learn as I was very busy with the children."

Eventually she was able to begin taking classes and now speaks good conversational English.

But even with classes, it can be a long process to pick up the language.

Start young

Age is the most important factor in language acquisition, says Mila Vulchanova, professor of linguistics at the Norwegian University of Science and Technology.

"There is a sensitive period in language learning, which is biologically determined, with an onset at birth and a decline around puberty. So the younger the immigrant, the better. Since this decline is only gradual, teenagers are at an advantage over adults.

"The ability to learn is believed to be determined by a competence called 'language aptitude'. Its underlying mechanisms are hotly debated. In a recent experimental series where we tested 100 Norwegian children aged 10, we found very high correlations between their short-term working memory, their native language scores, and their scores on English as a second language.

"Quite often a non-native speaker can be revealed by errors in pronunciation. This is due to the fact that the sensitive period for speech perception is very short. It ends when we are only eight or nine months old. Only bilinguals retain this for longer."

There are a number of systems for grading English. The government expects immigrants to reach "ESOL Entry 3" or "B1 level", also called "Intermediate 1" in Scotland, before they can be granted citizenship. It's equivalent to being able to hold a confident conversation and - although the government does not have a target figure - it might take 360 hours of study to achieve.

George Osborne said in June following the spending review that welfare claimants who don't speak English will have their benefits cut if they fail to attend language courses.

On Tuesday, a Channel 4 documentary - Why Don't You Speak English? - looks at four immigrants who have struggled with English since arriving a year ago spend a week living with a family in the UK.

Pub manager Freddy Sipson who hosted Fabian, a Colombian immigrant for the Channel 4 programme, says his charge made progress but it would take much longer than a week. "When he came he spoke pidgin English. He improved over the week. In two or three months, if he was in the right environment I'd say he'd be capable of having a good conversation and getting himself around."

Huan Japes, deputy chief executive of English UK, a trade body for language colleges, says a rule of thumb is 360 hours - 120 hours for each of three stages - to get to the standard the government expects benefit claimants to reach.

But many of the people who attend courses are visiting students rather than people settling in the UK. Immigrants tend to have very varied levels of education.

"Using 120 hours [for each stage of English fluency] is a rather traditional approach to course book learning," says Dr Elaine Boyd, head of English language at Trinity College London. "If someone is really highly motivated, they can learn really quickly. It's common for children under the age of 11 to be very immersed and be fluent in about six months."

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Even simple tasks such as going shopping can be a challenge

Adults may be better at reading and writing to begin with. But children are faster to pick up speaking and listening, says Dot Powell, director of the British Council's ESOL Nexus. School plunges them into the new language, their brains are attuned to sounds and slang - such as putting "innit" on the ends of phrases - and teenagers need language skills to belong to the group, she says.

'I had no English when I arrived'

Sabina Begum, 20 (left), arrived in from Bangladesh in February 2012.

"I didn't know the English alphabet, I didn't understand what people were saying. I understand now and know how to use an Oyster card on the Underground. It took me four months before I could have a conversation."

Shilpi Akter, 35, is also from Bangladesh and came to the UK in May 2011. English was "a little bit of a problem" until she took lessons.

"I needed an interpreter to see the GP. But recently I went on my own. My plan is to get a job when I finish. Before I left Bangladesh I was a teacher."

They learn English at the Arbour, a community centre in East London

Philida Schellekens, a language consultant, says that when she researched English language learning in Australia a decade ago the figure of 1,765 hours was used. That could mean four years of classes. It signifies the standard needed to do a clerical job in an office.

In the UK the estimate of about 360 hours to move up a level is about right, she argues. But there is political pressure to decrease this, she argues. "Every (UK) government has been trying to bring it down. We've got to hold the line, learning a language is a time consuming business."

Every immigrant's experience of learning English is different.

Thura, 35, a doctor, fled Burma for political reasons and arrived in the UK in 2009. "At the beginning it was really difficult to understand. My reading was okay but not my speaking or listening." After three months he got a job in a care home, which is where he picked up most of his English.

He has since done a masters degree at University College London and now works in a hospital. What matters is not classes so much as being surrounded by English speakers, he argues. "It takes eight months to a year to learn it if you listen to proper English speakers," he says.

Ling's older son, who was eight when they arrived from China, is a different story. "After three months here he could speak very well." Arriving at a young age gives him a "big advantage" over her, she believes. "He is 13 now and speaks like a British person."

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Seeking medical help can be daunting

A key factor for all immigrants is their level of education, says Powell. It will vary from illiteracy to degree level and have a large bearing on how they can learn in the classroom. "The other factor is if you already speak two or three languages you'll learn the next one faster," Powell says.

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Fabian features in the Channel 4 documentary

One thing most experts agree on is that exposure to hearing English is crucial. If someone is working only with people of their own nationality, or at home not meeting English speakers, they will not learn quickly.

And yet immigrants arriving on a spouse's visa are not eligible for free English classes for at least the first year. "You're almost on your own for the year unless you've got a spouse who'll pay," says Powell.

Is there a cut-off point after which you will never truly master the language?

Ling is still hopeful she can speak perfect English like her son. But Thura says pronunciation is a big problem for even confident speakers. "It's almost impossible for me to lose my accent. But for someone coming here at 10 years old it's possible."

Other people's accents are also a big barrier. "I support Manchester United but I can't understand Sir Alex Ferguson. Scottish is a really strong accent."